Using four-wave mixing in a 35-cm highly nonlinear bismuth-oxide fiber incorporated in an erbium-doped fiber laser, a stable dual-wavelength output is obtained. The spectral spacing has been tuned from 1.3 to 7.2 nm with a tunable fiber Bragg grating. Simultaneous tuning of the two wavelengths over 20 nm is also demonstrated using a tunable bandpass filter together with a birefringent filter defining a 100-GHz frequency comb. The output stability has been experimentally analyzed. An abrupt reduction in the intensity fluctuation is observed when the amplifier output power reaches 22.0 dBm. At 22.3 dBm, the fluctuation attains a lower limit of ∼1 dB.
©2007 Optical Society of America
Dual-wavelength laser sources are important in a wide range of applications including the generation of high-bit-rate soliton pulses , the differential absorption measurement of trace gases , the photonic generation of microwave carriers , and the realization of microwave photonic filters . The erbium-doped fiber (EDF) ring laser is an attractive candidate to generate dual-wavelength output since it provides a large gain, a high saturation power, and a relatively low noise figure. However, owing to homogeneous gain broadening and unstable mode competition in the EDF, dual- or multi-wavelength lasing is inhibited at room temperature. Different approaches have been proposed to solve the problem like the cooling of the EDF in liquid-nitrogen , the use of a frequency shifter in the cavity , the incorporation of a semiconductor optical amplifier for self-saturation , the application of polarization hole burning in overlapping fiber cavities , the use of DFB fiber laser source , the combination of Brillouin gain and EDF gain in the laser , and the introduction of four-wave mixing (FWM) in a nonlinear fiber inside the cavity [11–12].
The FWM effect is sufficiently large in about 50 to 100 m of a highly nonlinear photonic crystal fiber to support multi-wavelength lasing in an EDF laser [11–13]. To further reduce the size of the setup and to improve its stability against environmental changes, one can make use of a very short fiber segment with an ultrahigh nonlinearity to support the FWM. Recent development in highly nonlinear bismuth-oxide fiber (Bi-NLF) has resulted in a nonlinear coefficient of over 1360 (W∙km)-1 using a conventional step-index guiding structure . In this work, we demonstrate the use of a 35-cm Bi-NLF to support dual-wavelength oscillation in an EDF laser. The Bi-NLF has an extremely large nonlinear coefficient of ~1100 (W∙km)-1 at 1550 nm. The propagation loss and the core diameter are 0.8 dB/m and 1.6 μm, respectively. The group velocity dispersion is 260 ps/nm/km at 1550 nm and the dispersion remains to be normal for the whole C-band. Compared to the previous demonstrations of using a photonic crystal fiber, the length of the nonlinear fiber has been substantially reduced by two orders of magnitude. The work thus generates much practical interest towards the realization of a compact dual-wavelength EDF laser. In addition, the short fiber segment gives rise to a significant increase in the SBS threshold and allows the use of very intense light to introduce four-wave mixing.
We first use two fiber Bragg gratings (FBGs), one with a fixed and the other with a tunable reflection peak, to define the lasing wavelengths in the experiment. A tunable spectral spacing from 1.3 to 7.2 nm has been obtained in the dual-wavelength output. Next, to tune the dual-wavelength output while maintaining a fixed spectral spacing, we apply a 100-GHz birefringent comb filter in place of the FBGs. The filter defines a periodic set of spectral components with uniform transmission without relying on an array of FBGs  or a sampled Bragg grating . The tuning is performed by adjusting the spectral gain profile using a tunable bandpass filter in the setup. The dual-wavelength output has been successfully tuned from a center position of 1547.3 to 1568.7 nm over a range of 21.4 nm. We compare experimentally the stability of the lasing outputs with and without the Bi-NLF. It is observed that with the FWM, the laser operation is transformed from a hopping state with one to three-wavelength output to a stabilized dual-wavelength oscillation with ≤ 1dB intensity fluctuation. Quantitative analysis on the output stability is also performed as a function of the strength of FWM. We find that the intensity fluctuation shows an abrupt reduction from ∼10 to 4 dB at 22.0 dBm EDFA output power in our setup. At 22.3 dBm, the fluctuation reduces to a limiting value of ∼ 1dB.
2. Experimental setup
Figure 1 shows the setup of our dual-wavelength laser consisting mainly of an EDFA to provide the optical gain, a 35-cm Bi-NLF to support FWM, and a wavelength selective element in the cavity. The latter is composed of either two pieces of FBGs or a birefringent loop mirror filter (LMF) together with a bandpass filter. In the case of FBGs, a variable optical attenuator is placed between the two FBGs to equalize the power being reflected by the gratings. The LMF contains a 10-m polarization-maintaining fiber (PMF) with a birefringence of 3×10-4, giving rise to a comb spacing of 100 GHz, or 0.8 nm at ∼1550 nm. The transmission curve is shown next to the LMF in Fig. 1. The periodic and uniform transmission characteristic supports broadband multi-wavelength lasing of the fiber laser. An 8-nm tunable bandpass filter is used to control the spectral gain profile in the laser cavity. The dual-wavelength source is obtained at the 10% output port of a 90:10 fiber coupler. The EDFA used in the experiment is obtained from IPG (model EAU-2-TB) with a maximum output power of 33 dBm.
3. Results and discussion
First, two serially connected FBGs are used as the wavelength selective elements. One FBG has a fixed reflection peak at 1548 nm and the other has a tunable reflection peak from 1545 nm to 1555 nm. The tuning is achieved by adjusting the mechanical strain applied to the grating. Hence, the spectral spacing of the two wavelengths can be tuned.
When a relatively weak EDFA output power is used, the dual-wavelength output is unstable owing to gain competition between the two selected wavelengths. However, as the EDFA output power is increased to provide 22.3 dBm input to the Bi-NLF, a stable dual-wavelength output can be obtained. The enhanced stability is explained by a dynamic gain flattening process through the degenerate FWM effect. At the beginning, the two selected wavelengths will have unequal power (e.g. Pω1 > Pω2 due to gain competition caused by homogeneous gain broadening in the erbium-doped fiber. However, degenerate FWM at the Bi-NLF will lead to energy transfer from the higher power component to the lower power one . The flattening is brought about by an exchange of power between the two selected wavelengths through the continuous annihilation and creation of photons. The optical spectra of a series of dual-wavelength outputs are displayed in Fig. 2. In the experiment, the spectral spacing has been tuned from 1.3 to 7.2 nm.
In addition to the two wavelengths that are defined by the FBGs, two side modes are observed in the optical spectra, thus confirming the presence of four-wave mixing in the 35-cm Bi-NLF. The side mode suppression ratio increases from 31 to 42 dB as the wavelength spacing increases and causes a weaker FWM. When the output spacing is 1.3 nm, higher order FWM output can also be observed and a suppression ratio of 60 dB is recorded.
Apart from tuning the spectral spacing, it is also of interest to tune the emission wavelengths while maintaining a constant spacing between the outputs. Here, we use an all-fiber birefringent LMF that exhibits a wideband periodic transmission characteristic to define a 100 GHz (∼0.8 nm at 1550 nm) spectral grid. Owing to the wideband periodicity, we can incorporate an 8-nm tunable bandpass filter (BPF) in the setup to tune the spectral position of the selected wavelengths.
Figure 3 shows the tuning of 0.8-nm spaced dual-wavelength output with a side mode suppression ratio of over 30 dB. Higher order FWM output has been observed with a suppression ratio of 55 dB. The output power of the EDFA is set to 22.5 dBm such that only two dominating wavelengths are obtained at the output of the ring laser. By increasing the EDFA output power, more lasing wavelengths can be obtained. The center position of the output has been tuned from 1547.3 to 1568.7 nm, giving rise to a tuning range of 21.4 nm. During the tuning, the power difference between the two lasing wavelengths is observed to be less than 3 dB. The maximum tuning range is determined by the gain profile of the EDFA and the tuning range of the BPF. To further enhance the extent of tuning, a gain-flattened wide-band EDFA can be employed in the setup.
To understand the role of the 35-cm Bi-NLF, we further study the stability of the fiber laser source. By setting the EDFA output power to 22.5 dBm, a stable dual-wavelength source is obtained. Figure 4(a) depicts typical samples of the optical spectra captured at different instances over a time interval of 3 minutes. Intensity fluctuations at the individual wavelengths are found to be less than 1 dB. Higher order FWM components are also generated with a suppression ratio of 55 dB. When the Bi-NLF is removed from the setup, the output spectrum becomes very unstable. Under the same EDFA output power, we again capture four optical spectra as shown in Fig. 4(b) in a 3-minute interval. It is observed that the laser can oscillate instantaneously at one, two, or three wavelengths. Thus, it is concluded that without the Bi-NLF, the laser output becomes very unstable.
Stabilization of the dual-wavelength source is not simply caused by the presence of the Bi-NLF, it also depends on the strength of FWM. With the use of the Bi-NLF, Fig. 5 shows the maximum intensity fluctuation of individual wavelengths measured over a time interval of 1 minute at different EDFA output power. When the power is below ∼22.0 dBm, the fluctuation is relatively large and is over 10 dB. At ∼22.0 dBm power, we observe an abrupt drop in the intensity fluctuation to about 4 dB. With a further slight increase of the EDFA power, the fluctuation continues to drop until it reaches a limiting value of ∼ 1 dB at 22.3 dBm power. Our result thus shows that even with the Bi-NLF, certain extent of FWM is needed to maintain the stabilization of the source.
With the above setup, simultaneous lasing at more than two wavelengths can be supported. When FBGs are used as the wavelength selective elements for multi-wavelength oscillation, additional FBGs and VOAs are needed according to the desired number of output wavelengths. Owing to the difference in their reflectivities, precise control of the optical attenuation is required between the FBGs to achieve a stable multi-wavelength output. Unlike FBGs, a birefringent LMF offers the flexibility to increase the number of lasing wavelengths while keeping the simplicity of the setup. With a longer Bi-NLF or a higher EDFA output power to strengthen the FWM, additional wavelengths can be generated and stabilized through the power exchange process brought by the creation and annihilation of photons. By further increasing the EDFA output power to ∼27 dBm to enhance FWM in the Bi-NLF, we observe the generation of up to four stabilized lasing wavelengths in our setup.
Apart from the use of a tunable FBG, the spectral spacing of the output can also be controlled with a spacing-tunable birefringent comb filter . Furthermore, fast and precise tuning of the exact wavelength positions can be anticipated by incorporating an electro-optic phase modulator in the LMF  to adjust the overall birefringence.
A 35-cm highly nonlinear bismuth-oxide fiber has been successfully used to stabilize a dual-wavelength fiber laser at room temperature through the degenerate four-wave mixing effect. Tuning of the spectral spacing and the positions of the output wavelengths have been demonstrated using a tunable fiber Bragg grating and a birefringent loop mirror filter together with a tunable bandpass filter, respectively. The stability of the laser has been characterized by measuring the intensity fluctuation of the output both in the presence and the absence of FWM. It is observed that the output stability improves abruptly when the EDFA output power is increased to ∼22.0 dBm in our setup. A minimized intensity fluctuation of ∼ 1 dB is achieved when the amplifier output power reaches 22.3 dBm.
The work described in this paper is supported by the Research Grants Council of Hong Kong (CUHK 415705). The authors would like to thank Dr. Sugimoto and Dr. Ohara of Asahi Glass Co., Ltd. in providing the highly nonlinear bismuth-oxide fiber.
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